Method of and apparatus for bleaching and refining pulp



C. B. THORNE METHOD OF AND APPARATUS FOR BLEACHING AND REFIN'ING PULPFiled May 6, .1932

C B. Theme plant andat and improve Patentedian. 29, 1935 METHOD OF ANDare 'rns roa mason G REG lP Carl Busch Theme, Hawkesbury, Qatari,Ca'nada Application This invention relates to methods of and apparatusfor bleaching and refining pulpby means of chlorine gas, chlorinatedwater, or'the like.

The invention has for object to provide an improved method of increasedefliciency, economy, and safety, and is particularly, though notexclusively, applicable to the treatment of wood pulp. By means of theinvention a considerable saving'of bleaching medium is effected, and theprocess is continuous and easily controlled. It enables a gaseousbleaching medium to be. used with complete safety; the apparatusrequires much less space than ordinary low density bleaching apparatus,the time required'for bleaching is greatly reduced,,and an improvedproduct is produced.

A further object of the invention is to provide in a process of thischaracter an absorption zone, in which the preliminary absorption periodmay be varied at will, and provide for the initial penetration of thepulp by the bleaching medium.

Another object is to provide for maximum efficiency in the step of theprocessinvolvingthe treatment 'of low density pul A still further objectof the inventicn is to provide for the utilization of-all the-gases ofthe bleaching medium, avoiding their escape into the the same timeutilizing them to treat the white waterso that it may be reused in theprocess. 1

The accompanyi g drawing is a vertical sectional elevation andillustrates diagrammatically, and by way of example, one form ofapparatus for carrying the invention into effect, the iull line arrowsindicating the directionof flow of the pulp and the dotted arrows thegases.

1 indicates a supply of chlorine gas or chlorine water. The low densitypulp to be bleached enters the apparatus through the pipe 2 and flowstogether with a measure quantity of chlorine gas or chlorine water intoa mixer 3 where the pulp and bleaching medium are thoroughly mixed, 4represents a tower provided with an upper chamber 5 and lower chamber5'. An agitator 6 rotates in the chamber 5 to assist in keeping thefibres in suspension in the liquid; A scraper arm '1 is located atthebottom or the tower 4 and rotatesat a very slow speed to assist inthe removal of the pulp from the tower. The sraper arm '1 is driven byany suitable means, such as a shaft 8, which is connected to thespeed-reducer 9.

The pulp which is red to the mixer 3 is at low density, for example,between 3% and 6%. When it leaves the mixer 3 it flows down a channel 10intothebottomoithehamherSinwhichit 32, Serial No. 609,732

gradually rises and flows over into a channel 11 and so down to thelower chamber 5 of the tower. Means are preferably provided forcontrolling the level to-which the pulp can rise, and thus the time thatthe pulp will remain, in the chamber 5. As illustrated, these meanscomprise a vertically adjustable gate valve or'weir 12, controlled, forexample, from above by means of a hand wheel 13. The pulp then graduallyflows downward in the chamber 5' and flows out through a chute 14 to atank 15.

It is desirable to keep the level of the pulp in the chamber 5 as nearlyconstant as possible, and suitable means are provided for this purpose,for example, a valve 16 is placed in the chute 14 and this valve isautomatically controlled by a float 1'1 which is connected to the valve16 by a weighted arm 18 and a cable 19 guided by pulleys 20. It will beseen that as the level of the pulp in the chamber 5' drops, the float1'1 will also drop and thus wholly or partially close the valve 16 andretard the flow of the pulp through the chute 14 and so cause the pulpto rise-again in the tower to the desired level.

A pipe 21 is provided through which caustic or any other chemical may beadded if desired. The pulp from the tank passes through a pump 22,through a pipe 23 up to a washer and thickener 24 where it is washed andconcentrated to a high density of say, 15% or more. From the-thickenerthe pulp drops down a chute intoa mixer 25. Bleach medium from the tank26 is also fed to the mixer 25 and the pulp and bleaching medium arevery intimately mixed before dropping into a high density tower 2'1.This high density tower may conveniently be of the kind described inapplicants United States Patent No. 1,642,978, dated September 20, 1927.1

The pulp gradually descends in the tower 2'1 tothe bottom openingthereof. A scraper arm 28 scrapes the pulp towards the centre opening sothat the pulp is discharged uniformly from across the whole diameter ofthe tower. pulp is preferably diluted by water from a pipe 29 and iswashed out through a spout 30 into a tank 31 from which the pulp isdelivered by a pump 32 to a washer 33, and thence to any known system oflow densitybleaching indicated diagrammatically at 34.

It will be seen that the above describes a threestage system ofbleaching in which the first stage is at low density and the bleachingmedium is either chlorine gas or chlorine water. The sec- The The thirdstage is the customary low density system using a hypochlorous orsimilar bleaching medium. The chemical treatment which takes a bleachingmedium in a continuous system of bleaching has been the difiiculty ofvconfining the gas and therefore making it safe to operate. It will benoticed, however, that, according to the present invention, the chamber5 forms a perfect seal for the gas andthat the system is entirely closedso that there can be no danger from escaping gases. Some gas will, ofcourse, be ,given oil in both the towers 4 and 27 and pipes and 36 areprovided to convey these gases to a recovery system. The excess gaseswhich are sucked from the towers and through the recovery system by afan 37, flow through the pipes 35 and 36 into the bottom of a recoverytower 38. recovery tower is filled above its perforated false bottom 39with a filling such as acid resistant glazed tile. At the top of thetower is a spray 40 which sprays water on to the tile. The gases rise inthe tower 38 and are absorbed by the water which is trickling down overthe tile. The resultant hypochlorous solution flows through a pipe seal41 'into a box 42 from where it is returned to the spray 40 through apump 43. When the hypochlorous solution has reached the desiredconcentration, it is bled through a pipe 44 and is led to any desiredpoint ofconsumption, for instance, to the bleach liquor box 26.

It is advantageous to use the spray white water down the tower 38. Thewhite water thus treated is rendered fit for reuse in the mill processdue to chlorine gas combining with the slimy ingredients in the whitewater.

It will also be obvious that the chamber 5 and its associated parts neednot necessarily be placed in the tower 4, but may be constructed as aseparate unit located at any convenient point,

so long as the passage 11 communicates with the lower chamber 5'.

It will be understood that the steps in the process can be varied ifdesired in order to suit the particular conditions required by" certainclasses ofpulps. For instance, the high density stage may be used'firstfollowed by the low density using chlorine gas or chlorine water andthen bleaching medium is fed into the lower end of a closed absorptionzone, maintaining the mixture in said zone for a predetermined period oftime to permit absorption by the pulp of the bleaching medium, gentlyagitating the mixture substantially only adjacent the bottom of saidzone to. avoid segregation of fibre and to maintain the upper portion ofsaid mixture in a substantially unagitated state, and in the secondstage'discharging the mixture into a reaction zone, maintaining themixture therein in a quiescent state for a predetermined period of timeand then dischargng the same from said reaction zone.

2. A continuous two-stage method of bleaching low density pulp whichcomprises feeding a mixture of said pulp and a bleaching agent into thelower end of an absorption zone, closed to the atmosphere, so as toavoid agitation of the upper portionof the mass in said zone,maintaining the bulk of said mass in a substantially unagitated'absorption zone and wherein the period of absorption is varied bychanging the depth of said absorption zone. r

4; Apparatus for bleaching pulp comprising, in combination a mixer forpulp and bleaching agent, a chamber providing a substantially quiescentabsorption zone and a second chamber providing a quiescent reactionzone, both said zones being closed to the atmosphere, a conduit forfeeding a mixture of pulp and bleaching agent from said mixer to thebottom of said absorption zone, a variable reaction weir, adjacent thetop of said discharge zone, for feeding said mixture directly to theupper portion of the reaction zone, a discharge outlet in the bottom ofsaid reaction zone and means in each-of said zones, operative adjacenttheir bottoms only, for preventing undue segregation or. accumulation offibre at the bottoms of said chambers. p

5. Apparatus for bleaching pulp comprising a mixer for pulp. andbleaching agent, a bleaching chamber including an absorption zone initsupper end and a reaction zone in its lower end, a conduit leading fromsaid mixer into near the bottom of said absorption zone, said mixerdischarging through said conduit into the bottom of said absorption zonewhich is provided with an outlet at its top, a second conduit connectingsaid outlet with the upper part of said reaction zone and adapted fordischarging into the top of said reaction zone and means for preventingaccumulation of fibre in the bottom of each of said zones.

' CARL BUS CH THORNE. 5

